The kidney has been shown to be a site of production of IGF-I in the rat, mouse and human in many experimental conditions. IGF-I mRNA and peptide were found to increase in renal compensatory hypertrophy following partial nephrectomy suggesting a role of this peptide in renal growth. However the sites of action and the cell types responsible for IGF-I synthesis have not been identified. To further investigate the role of IGF-I in the renal glomerulus we used mouse glomerular cells in culture. Separate cultures of glomerular endothelial, epithelial and mesangial cells were obtained from isolated glomeruli using patch cloning and dilute plating techniques. These cells were studied for the presence of IGF-I receptors, the mitogenic response to IGF-I and the synthesis of this growth factor. Glomerular mesangial cells were found to have IGF-I receptors and to be sensitive to the mitogenic action of IGF-I. These cells were also found to synthesize and release in the culture medium an IGF-I like molecule. Glomerular endothelial and epithelial cells were found to have IGF-I receptors, but no IGF-I like molecule was detected in their culture medium. These data suggest a role for IGF-I in the maintenance and regulation of kidney structure and function. They demonstrate that glomerular mesangial cells may be a source of IGF-I in the kidney. IGF-I might be one of the growth factors locally released in the glomerulus acting in an autocrine and paracrine fashion.